1. Field of the Invention
The present invention relates to a surface-mounted over-current protection device and more particularly, to a surface-mounted over-current protection device exhibiting a small covered area, high hold current, and positive temperature coefficient (FTC) behavior.
2. Description of the Prior Art
The resistance of a PTC conductive composite is sensitive to temperature change. When a PTC device containing PTC conductive composite operates at room temperature, its resistance remains at a low value so that the circuit elements can operate normally. However, if an over-current or an over-temperature situation occurs, the resistance of the PTC device will immediately increase at least ten thousand times (over 104 ohm) to a high resistance state. Therefore, the over-current will be counterchecked and the objective of protecting the circuit elements or batteries is achieved. Because the PTC device can be used to effectively protect electronic applications, it has been commonly integrated into various circuits to prevent over-current damage.
In general, the PTC conductive composite contains at least one crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer(s). The crystalline polymer is mainly a polyolefin polymer or a fluoropolyolefin polymer such as polyethylene, polyvinyl fluoride or polyvinylidene difluoride (PVDF). The conductive filler(s) is mainly carbon black.
The conductivity of the PTC conductive composite depends on the content and type of the conductive fillers. In general, the resistivity of the PTC conductive composite containing the carbon black as the conductive filler seldom reaches below 0.2 Ω-cm. Even though the low resistivity mentioned above is achieved, the PTC conductive composite often loses the characteristic of voltage endurance. Therefore, a conductive filler, which is different from carbon black, with lower resistance should be used in the to PTC conductive composite to reach a resistivity below 0.2 Ω-cm. Since the conductivity of carbon black is relatively low (i.e., relatively high resistance), if carbon black is applied to a surface mount device (SMD) with fixed covered area, the hold current of the SMD is limited to certain level due to the resistance limitation of carbon black. The hold current mentioned above means a maximum current the PTC device can endure without trip at a specific temperature.
Although a multi-layer PTC structure could be used to increase the hold current, SMD device performance is eventually limited due to the limitation of total height as well as the number of PTC layers of the SMD device. In general, for a single PTC layer of carbon black filled SMD over-current protection device, the ratio of the hold current to the area of a PTC material layer should not exceed 0.16 A/mm2. The SMDs currently available on the market have a certain shape characterized by the width and the length, which are defined as a form factor in the specification of the SMD. Consequently, the length and width of the SMD determine the covered area of the SMD. For example, SMD1812 indicates a SMD with a length of 0.18 inches and a width of 0.12 inches and thus a covered area of 0.18″×0.12″, equivalent to 4.572 mm×3.048 mm (i.e., 13.9355 mm2) in metric system. For an SMD 1812 equipped with an over-current protection device using carbon black as the conductive filler, a single PTC material layer hardly reaches a hold current of 1.8 A. If the SMD1812 having two PTC material layers can hold a current of 3.6 A, the hold current per unit covered area per PTC material layer can be calculated as: 3.6 A/(2×13.9355 mm2)=0.129 A/mm2, which is below 0.16 A/mm2. Therefore, it is highly desirable that a new type of SMD device could be developed to exceed the 0.16 A/mm2 barrier.